The primary purpose of the tread is to provide traction with the road surface. Sufficient traction is particularly important in inclement weather when the road surface is covered with precipitation. To maintain traction in less-than-ideal road conditions, the tread is most often designed with a tread pattern. The tread pattern consists of one or more grooves or channels in the surface of the tire. The grooves may divide the tread into regions, often referred to as “lugs.” The grooves help the tread maintain traction with the road by allowing the tread to expel precipitation from between the lugs and the road surface. Traction is also dependent upon the composition of the tread. Compositions with good traction are known, but simply selecting a composition for improved traction can have negative repercussions on the other characteristics of the tire, as set forth below, such as the wear resistance and rolling resistance.
The rolling resistance of the tire influences the fuel efficiency of the vehicle. It is desirable to reduce the rolling resistance to increase fuel efficiency. Generally as a tire wears, its rolling resistance decreases because the mass of the tire decreases. Like traction, rolling resistance is also dependent upon the composition of the tire. However, while compounds having low rolling resistance are desirable for improved fuel efficiency, those same compounds often do not provide the necessary minimum traction. The reverse is also true. That is, a compound with good traction properties typically exhibits an unacceptably high level of rolling resistance.
In addition, another characteristic of the tread is its wear resistance. The useable life of a tire is generally determined by the number of miles that can be driven on the tire before the tire tread becomes unacceptably thin, which is dictated by safety concerns. For this reason and others, it is desirable for the tread to resist wear. The compound from which the tread is made also determines the resistance of the tire to wear. However, similar to the competing characteristics between rolling resistance and traction characteristics of the tread, set forth above, compounds that are ideal for reducing treadwear typically exhibit poor traction.
To complicate matters, the rolling resistance and traction of a tire tread change as the tread inevitably wears away—there is a dynamic relationship between these characteristics. For instance, the rolling resistance of the tire generally decreases as the tread wears simply because of the simultaneous reduction in the weight of the tread. Furthermore, when the tread wears away or thins, the volume of the tread pattern is reduced. Since the tread pattern is primarily responsible for allowing the tread to maintain traction on wet or snow covered roads, reduction in the thickness of the tread, and the consequential loss in the void volume and siping depth of the tread pattern, reduces the traction of the tread in these circumstances. Therefore, where rolling resistance generally decreases, which is a desirable trend, the traction of the tread in inclement weather also decreases, which is an undesirable trend.
As a result, it is common practice to construct a tread of a compound that is essentially a tradeoff between traction, rolling resistance, and treadwear. The application for the tire may dictate whether any one of these characteristics is to be emphasized. However, where one of the characteristics is emphasized, the remaining characteristics of the tire can be less than optimum.
Therefore, there remains a need for tire treads that exhibit desirable combinations of traction, rolling resistance, and treadwear characteristics that can be maintained throughout the useable life of the tire.